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Just curious about what people have their 250hp manual transmission non-A/C cars, “warmed up” curb idle speed set to. I know what the manual says but what have owners found to be practical on the street for the WCFB on these motors ?
I think with todays gas you have to increase the idle speed on ALL old cars. At least that has been my experience. The factory setting are OK when it is cold/just warming to operating temps but after you drive the car a little it becomes to slow and will often die if you let off too quickly etc.
So far, folks are confirming what I've found ... much below 750 RPM things don't seem stable....anything above that and the car starts "dropping shots" and I can't tune out the slight burbles with the mixture screws.
I think maybe the centrifugal weights are coming on above that RPM...
So far, folks are confirming what I've found ... much below 750 RPM things don't seem stable....anything above that and the car starts "dropping shots" and I can't tune out the slight burbles with the mixture screws.
I think maybe the centrifugal weights are coming on above that RPM...
The secret is you have to find the sweet spot were it will idle like you want to keep from dying for little reason and dieseling where you have to shut it off and release the clutch a little in gear with you foot on the brake to keep it from running on.
My dual wcfb carbs seem happiest between 750-800 rpm. I have tried slowing it down to around 650, but I can’t get a very smooth idle. I’m convinced gas quality affects idle quality a lot as well.
So far, folks are confirming what I've found ... much below 750 RPM things don't seem stable....anything above that and the car starts "dropping shots" and I can't tune out the slight burbles with the mixture screws.
I think maybe the centrifugal weights are coming on above that RPM...
It’s a saying for a slight sporadic idle miss. It happens out both exhaust pipes. The car performs flawlessly otherwise in all regards. I will make a recording of it maybe after the holiday (and my guests leave).
It’s a saying for a slight sporadic idle miss. It happens out both exhaust pipes. The car performs flawlessly otherwise in all regards. I will make a recording of it maybe after the holiday (and my guests leave).
Sorta like a putt, putt, PUTT, putt, putt.
Have you tried to disconnect the vacuum advance and idle the car with 6-8* initial timing just to see if the extra heat helps the idle combustion. On my 67 300hp engine it seems better when you take some of the timing out. Also try the engine with your thumb over the PCV valve and see if the extra idle A/F that's pulled through the idle circuit helps richen the mixture. I am not saying drive the car without a PCV but if you feel the idle gets better that may give a direction to go because the fuel is different today.
FWIW, also keep in mind that the almost closed throttle blades with high humidity can form ice which can upset the idle flow. That was the initial reason of the hot slot holes under the primary throttle blades. It's hard to imagine on a hot motor this would be an issue but the speed of the air around the blades is very fast which may be why when the blades are opened a bit more the idle does not skip and misfire.
FWIW, also keep in mind that the almost closed throttle blades with high humidity can form ice which can upset the idle flow. That was the initial reason of the hot slot holes under the primary throttle blades. It's hard to imagine on a hot motor this would be an issue but the speed of the air around the blades is very fast which may be why when the blades are opened a bit more the idle does not skip and misfire.
That's why all carbureted airplane engines have carburetor heat controls to avoid carb ice-up and engine stalls at very low power settings and flight idle on final approach; airplane carb ice will almost always form when ambient air is well above freezing due to very high air velocity at near-closed throttle conditions.
I think with todays gas you have to increase the idle speed on ALL old cars. At least that has been my experience. The factory setting are OK when it is cold/just warming to operating temps but after you drive the car a little it becomes to slow and will often die if you let off too quickly etc.
This is exactly what I have found. Even fully warmed up and adjusted to 750, as I drive around I see the rpms drop to 600 or less sometimes almost stalling at traffic lights. I’m now at 850 and will see how it does driving around.
This is exactly what I have found. Even fully warmed up and adjusted to 750, as I drive around I see the rpms drop to 600 or less sometimes almost stalling at traffic lights. I’m now at 850 and will see how it does driving around.
Since you’re in Texas I would think you would have access to ethanol free gas? I know on my recent trip through Texas there were many places there and in Oklahoma that I could buy it. That will fix a lot of your problems if you use it.
My distributor was completely overhauled by Lars not that long ago so I'm gonna give that a pass as an issue. I also put new plugs and wires on the car last week and the problem persists. A bit strange because I get the slight "miss" at 750 RPM but when the Vintage Air compressor kicks in the idle drops to a tad over 600 and smooths out. To wit, the problem abates under load apparently.
I have a fresh 3501 WCFB I overhauled on the shelf I may swap onto the car later in the month when my guests depart just for a check. I will try tbarb's suggestions though. Based on Lar's graph he provided me...anything north of 800 RPM will start up the advance curve (e.g. the distributor weights come into play).
The car never stalls but you can feel the "shots" while idling and its my only complaint with the car's performance... I don't want to back the idle down much more from 750 or I'll have stability issues when the Vintage Air compressor kicks in.
I'll have to cogitate some on the mixture icing as its sucked down the bore in the summer in Orlando - talk about counter-intuitive ?
Last edited by Frankie the Fink; 09-04-2018 at 07:02 AM.
'66 L79 4 speed..... I have set between 750-800. I run 93 octane with 10% ethanol Shell gas. No problems. In very hot weather I'll get a little "run on" when I turn the car off, so I sometimes do the clutch trick that 68hemi mentioned.
My '65 250 hp engine idles around 600 or less just fine. My '63 360 FI car will idle fine at 725 but will stall, sooner or later at 700. I keep it at 750.
89 octane E 10. Is my tach wrong or do we have a higher proof ethanol here?
Frank, I too have experienced the "dropping shots" issue on my base engine '67, and for the longest time I thought it was an ignition issue. I finally was able to improve idle smoothness by running the car with the top ignition shield removed. It was my thinking that, even though there is a thin Mylar elec. insulator strip on the inside top of the shield, that occasionally a plug wire would "short out" to the shield. I don't have any hard data to prove the theory other than the car seems to run smoother without the shield in place. I also shortened the coil-to-cap wire by about 2 inches so that runs under the other plug wires. Before I did this that wire ran right against the top shield. Anyway, something else that you might want to check into if you haven't already.
Phil
'66 L79 4 speed..... I have set between 750-800. I run 93 octane with 10% ethanol Shell gas. No problems. In very hot weather I'll get a little "run on" when I turn the car off, so I sometimes do the clutch trick that 68hemi mentioned.
Hi, my L-79 needs 800-900 rpms for stability at idle...I find depending on T & H...I have to turn my idle screw 1/2 turn faster in high T & H and 1/2 turn less...as air drys out and temp drops. It is just the way it is......for the last 350,000 miles.........& 50 years.......
Hi, my L-79 needs 800-900 rpms for stability at idle...I find depending on T & H...I have to turn my idle screw 1/2 turn faster in high T & H and 1/2 turn less...as air drys out and temp drops. It is just the way it is......for the last 350,000 miles.........& 50 years.......
Jack
Jack:
I think by now that you KNOW your car. Agree with your analysis. My car is a 67 327/350 HP car. 800-900 RPM is a sweet spot for this engine.
I think some of you are having a problem distinguishing the difference between the natural rhythm of the engine and Sorta like a putt, putt, PUTT, putt, putt.
09-03-2018, 05:41 PM
Question on Curb Idle
Agree with your analysis. My car is a 67 327/350 HP car. 800-900 RPM is a sweet spot for this engine.